The next big cooperative European-US space mission will be to Europa, the ice-crusted moon of Jupiter.

A joint working team is being set up to consider what sort of spacecraft would be needed and what each side could do.

Officials in Washington and Paris are keen to follow up the spectacular success of Cassini-Huygens at Saturn.

"It was a beautiful marriage and we really are looking to do a repeat," said Professor David Southwood, from the European Space Agency (Esa).

Southwood told the BBC News website that "Europe could do Europa on its own", but that a cooperative venture was extremely attractive.

It's a natural for the next big international collaboration in space

Prof Fred Taylor, Oxford University

Many scientists agree that Europa is now a high priority target for a major mission.

The moon, discovered by Galileo, is slightly smaller than the Earth's Moon. Its covering of white and brownish-tinted ice is riven with cracks that are probably the result of stressing caused by the contorting tidal effects of Jupiter's strong gravity.

Researchers speculate that tidal heating may even have produced vast oceans of water under the ice sheet and that this environment could harbour micro-organisms.

Convenient time

The Esa director of science held discussions about Europa with counterparts at the US space agency (Nasa) at the end of last week. "I've definitely piqued their interest," he said.

The discussions are at a very early stage - and a mission that would launch no earlier than 2016 is some way off becoming a reality.

Nevertheless, Professor Southwood said it was a good time to consider how the two agencies could build on their Saturn experience, which has produced stunning images of the ringed planet and put a lander on the surface of Titan.

EUROPA - MOON OF JUPITER

Orbit: 670,900km from Jupiter

Diameter: 3,138km

Discovered by Galileo and Marius in 1610

Ice crust may be many tens of km deep

The Americans had planned to go to Europa independently with their Jupiter Icy Moons Orbiter (Jimo). But the ambitious project, which would have used a nuclear propulsion system, has been shelved as Nasa re-focuses its budget on a White House initiative that could take humans back to the Moon.

As a consequence, the European suggestion of a joint mission to Europa has been favourably entertained.

As with Cassini-Huygens, Southwood envisages the new mission incorporating a double-spacecraft architecture.

Surface imperative

"You've got to have a relay satellite," he explained. "You go together; you fly out there in tandem.

"They separate after Jupiter orbit insertion and then you leave the relay satellite in orbit around Jupiter, preferably in a resonance with Europa.

The power of pictures: Huygens' image of Titan

"Then there's a debate about what you do at Europa. Personally, I would like deep-penetrating radar [on an orbiter]. But that's because I'm a remote-sensing man.

"I believe you get more by getting the global picture than you do by scratching and sniffing the surface."

But the pressure to go down to Europa's cracked and blotchy surface would be immense, said Professor John Zarnecki, the principal investigator on the surface science instruments loaded on to Huygens for its Titan descent.

"If it is technically feasible to go to the surface, you would want to do that. Huygens' surface image on Titan says everything," the Open University researcher enthused.

"But, it may be that what you want to do - to look below Europa's ice - you can do that better from orbit.

"The Esa-Nasa group that's going to be set up will look at just these sorts of technical issues," added Professor Zarnecki, who has been party to the initial trans-Atlantic discussions.

Power needs

Researchers at the German Aerospace Centre are already developing a prototype technology that could be used to melt through Europa's ice sheet. Any water might be a considerable (and possibly unreachable) way down - 20-30km down.

Once under the sheet, the probe would take samples and drop mass to begin a slow climb back up the ice column. On the surface, it could then send data to an orbiter or relay satellite for onward transmission to Earth.

Europe already has a major mission en route to Jupiter's orbit - the Rosetta mission, which will chase down a comet and put a lander on its surface. This has given Esa the confidence to go it alone to Europa if the Americans decide eventually not to participate in a joint mission.

But a key factor is likely to be power systems. Although solar panels will work on spacecraft at that distance, the desire for sufficient energy to drive many instruments means any mission would really need to go with radioisotope thermal generators (RTGs) - solid state electrical generators powered by the heat of radioactive decay.

Europe has no expertise with RTGs - the Americans have, and Cassini carries three to provide 700 watts to its systems.

"I'd much rather do this with RTGs," said Professor Southwood. "And that makes it almost certainly a joint venture with the Americans and why should we do it separately?

"This was waiting to happen. Someone just had to say it."

Professor Fred Taylor, of Oxford University, UK, said the case for going to Europa was compelling.

"The attraction of Europa is that it is a water world - the surface is frozen, of course, because of its exposure to cold space, but not far underneath the ice is an ocean of warm water.

"We have never explored such a place beyond our own Earth, and the technology required is not too different from the successful US-European Cassini-Huygens mission, so it's a natural for the next big international collaboration in space," commented the scientist, who worked on the 1990s Galileo mission to Jupiter.

"It will be much cheaper than Jimo, which is more of a long-term project (and which has not been abandoned completely)."